linux-kernel - Re: [PATCH v8 03/10] eventfd: Increase the recursion depth of eventfd

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Date:   Thu, 17 Jun 2021 16:33:40 +0800
From:   He Zhe <zhe.he@...driver.com>
To:     Xie Yongji <xieyongji@...edance.com>, mst@...hat.com,
        jasowang@...hat.com, stefanha@...hat.com, sgarzare@...hat.com,
        parav@...dia.com, hch@...radead.org,
        christian.brauner@...onical.com, rdunlap@...radead.org,
        willy@...radead.org, viro@...iv.linux.org.uk, axboe@...nel.dk,
        bcrl@...ck.org, corbet@....net, mika.penttila@...tfour.com,
        dan.carpenter@...cle.com, joro@...tes.org,
        gregkh@...uxfoundation.org
Cc:     songmuchun@...edance.com,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        He Zhe <Zhe.He@...driver.com>
Subject: Re: [PATCH v8 03/10] eventfd: Increase the recursion depth of
 eventfd_signal()



On 6/15/21 10:13 PM, Xie Yongji wrote:
> Increase the recursion depth of eventfd_signal() to 1. This
> is the maximum recursion depth we have found so far, which
> can be triggered with the following call chain:
>
>     kvm_io_bus_write                        [kvm]
>       --> ioeventfd_write                   [kvm]
>         --> eventfd_signal                  [eventfd]
>           --> vhost_poll_wakeup             [vhost]
>             --> vduse_vdpa_kick_vq          [vduse]
>               --> eventfd_signal            [eventfd]
>
> Signed-off-by: Xie Yongji <xieyongji@...edance.com>
> Acked-by: Jason Wang <jasowang@...hat.com>

The fix had been posted one year ago.

https://lore.kernel.org/lkml/20200410114720.24838-1-zhe.he@windriver.com/


> ---
>  fs/eventfd.c            | 2 +-
>  include/linux/eventfd.h | 5 ++++-
>  2 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index e265b6dd4f34..cc7cd1dbedd3 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -71,7 +71,7 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
>  	 * it returns true, the eventfd_signal() call should be deferred to a
>  	 * safe context.
>  	 */
> -	if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
> +	if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count) > EFD_WAKE_DEPTH))
>  		return 0;
>  
>  	spin_lock_irqsave(&ctx->wqh.lock, flags);
> diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
> index fa0a524baed0..886d99cd38ef 100644
> --- a/include/linux/eventfd.h
> +++ b/include/linux/eventfd.h
> @@ -29,6 +29,9 @@
>  #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
>  #define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
>  
> +/* Maximum recursion depth */
> +#define EFD_WAKE_DEPTH 1
> +
>  struct eventfd_ctx;
>  struct file;
>  
> @@ -47,7 +50,7 @@ DECLARE_PER_CPU(int, eventfd_wake_count);
>  
>  static inline bool eventfd_signal_count(void)
>  {
> -	return this_cpu_read(eventfd_wake_count);
> +	return this_cpu_read(eventfd_wake_count) > EFD_WAKE_DEPTH;

count is just count. How deep is acceptable should be put
where eventfd_signal_count is called.


Zhe

>  }
>  
>  #else /* CONFIG_EVENTFD */